Among image forming apparatuses of an electrophotographic type, there is an apparatus having a function for executing a trapping process for slightly overlaying boundaries of objects such as images, figures, characters, or the like of different colors or a resolution converting process for converting input resolution into resolution adapted to enable an image process. The trapping process is a process for preventing a blank (called a “white gap”) of a boundary between the image objects which are formed by the different colors and is a process for expanding an outline of one of the objects whose boundaries are in contact with each other toward the other object by a predetermined amount. Thus, an overlap of the objects occurs and “white gap” that is caused by a misregistration or the like can be prevented. The trapping process is not unconditionally executed but, generally, whether or not the process is executed is determined according to density of the objects whose boundaries are in contact with each other. This is because when the density of at least one of the objects whose boundaries are in contact with each other is thin, even if there is “white gap”, it is inconspicuous and when the density of both of the objects is thick, “white gap” is conspicuous. Besides them, a technique for designating the trapping on a page unit basis and a technique for changing applying conditions according to attributes of the objects have been proposed.
The resolution converting process is such a process that in the case where resolution of an image which is input by the user to the image forming apparatus of the electrophotographic type differs from resolution which can be processed by each image process such as color process, filter process, or the like, such resolution is converted into resolution which can be processed.
In the resolution converting process, a converting method is changed according to an attribute or density of an image serving as a target. For example, in Japanese Patent No. 04143519, with respect to an image of a character attribute, in the case where it is intended to convert the image of 1200 dpi into 600 dpi, a result obtained by weight-averaging the density of four peripheral pixels of a target pixel is converted into one pixel. Or, with respect to an image of a graphic attribute, by selecting one pixel of the maximum density among the four peripheral pixels, the image is converted into 600 dpi, or the like. The density of edges of such an image becomes uneven due to a weighting method or a phase. However, by executing such a resolution conversion, a data amount of the image after the resolution conversion can be reduced. Therefore, a load of each image process (color process, process for a character) which is executed after completion of the resolution conversion can be reduced. There has also been known a fact that when the resolution converting method is adaptively switched based on such an attribute, each image process is executed to the resolution converted image, after that, the resolution of the image is returned to the input resolution according to a certain rule, and the resultant image is output, the quality of the image is good.
Although the trapping process and the resolution converting process are executed in order to improve picture quality as mentioned above, if both of them are used, various troubles occur. For example, when the resolution converting process, particularly, the resolution conversion is executed by the weight or average, the density of the edges becomes uneven. After that, when the trapping process is executed, a mode to perform the trapping to the image of certain density or more is set. Thus, although the image inherently has the same density, by the resolution conversion which was performed before, the density conversion is executed to a portion whose edge density is equal to or less than the density adapted to perform the trapping and to a portion whose density is equal to or larger than the density adapted to perform the trapping, so that the trapping process is performed in places. Consequently, not only there is no effect of the trapping but also the portions where the density of the edges becomes uneven by executing the resolution converting process typically become conspicuous.
That is, in the regions where both of the processes such as trapping and resolution converting process have been performed, there is a case where the trapping is performed in places and the image deterioration is remarkable.